An energy storage device like a lithium-ion secondary battery had been used as a power source of mobile equipment such as a notebook computer or a mobile phone. The energy storage device has been recently applied to wider fields and has been used as a power source of an electric vehicle. Such an energy storage device requires a technique for accurate estimation of discharge capacity (battery capacity) in a certain deterioration state.
There have been conventionally proposed techniques for estimation of discharge capacity of an energy storage device (see Patent Document 1 and Non-Patent Document 1, for example). Patent Document 1 discloses estimation of battery capacity according to the law (linear law) that a battery capacity retention ratio decreases in proportion to a battery cumulative operating period. Non-Patent Document 1 discloses estimation of battery capacity according to the law (root law) that a battery capacity retention ratio decreases slowly as a cumulative operating period elapses.